1. (Technical Field)
The present invention relates to a novel nucleating agent or a direct positive silver halide photosensitive material which comprises the nucleating agent and a nucleation accelerator in combination, and to a method for forming a direct positive image.
2. (Prior Art)
A photographic technique which requires neither a reversal process nor a negative film for obtaining a direct positive image is well known.
Conventional methods of obtaining a direct positive image by using a silver halide photosensitive material are mainly classified into two types in view of their practical usefulness, except for a special type.
One of these types is the method which uses a previously fogged silver halide emulsion and destroys the fogged nuclei of an exposed portion (latent image) by employing solarization or Herschel effect, to obtain a direct positive image.
The other type is the method which uses an unfogged internal latent image-type silver halide emulsion and conducts surface development, after or during fogging treatment after image exposure, to obtain a direct positive image.
The above-described internal latent image-type silver halide photographic emulsion is defined as a silver halide photographic emulsion of a type in which sensitivity specks are mainly contained in the silver halide grains and latent images are mainly formed in the grains by exposure.
The second method offers generally higher sensitivity than the first method and is suitable for applications requiring high sensitivity. The present invention concerns the second method.
Various techniques have been known in this technical field. Typical examples include the techniques described in U.S. Pat. Nos. 2,592,250, 2,466,957, 2,497,875, 2,588,982, 3,317,322, 3,761,266, 3,761,276, and 3,796,577, and British Patent Nos.1151363, 1150553, and 1011062.
These known methods can provide photosensitive materials with relatively high sensitivity, as a direct positive type.
The mechanism of the formation of a direct positive image is described in detail in, for example, T. H. James, "The Theory of The Photographic Process" 4th Edition, Chapter 7, pp. 182-193 and U.S. Pat. No.3,761,276.
Specifically, it has been believed that a photographic image (direct positive image) is formed in an unexposed portion by selectively forming fogged nuclei only on the surfaces of the silver halide grains in the unexposed portion using surface desensitization due to so-called internal latent images which are produced in silver halide grains by the first imagewise exposure and then performing a so-called usual surface development.
Known means for forming selectively fogged nuclei, as described above, include a method generally called "light fogging method" which provides a second exposure on the entire surface of a photosensitive layer (as disclosed, for example, in British Patent No.1,151,363) and a method generally called "chemical fogging method" which uses a nucleating agent. The latter method is described in, for example, "Research Disclosure" Vol. 151, No.15162, pp. 76-78 (issued in November, 1976).